convertToType: don't allow None -> 'None' conversion
convertToType: don't allow None -> 'None' conversion

In case of a string None becomes 'None', which is not the intension
for convertToType. So handle the None case specifically. The to be
added AddSettings relies on being able to pass None.

While at it, never accept a min max for a string.

victronenergy/localsettings